1. Field of the Invention
The invention relates to a silicon wafer having a greatly reduced tendency toward oxygen precipitation, and to a method for producing the silicon wafer, the method comprising a thermal treatment.
2. Background Art
Silicon single crystals are usually pulled from a silicon melt situated in a quartz crucible by means of the Czochralski method. As a result of corrosion of the quartz crucible, oxygen passes into the silicon melt, and is incorporated into the crystal in concentrations of a few 1017 to a few 1018 cm−3 (atoms per cubic centimeter). The oxygen is initially present in dissolved form, but is supersaturated at room temperature and typical temperatures that prevail during the production of electronic circuits and components. Therefore, it precipitates during the production of electronic circuits and components or other thermal treatments at similar temperatures. So-called BMDs arise in the process. These are oxygen agglomerates with or without additional defects which can arise during the thermal treatments directly in the vicinity of the oxygen agglomerates. Nuclei for the BMDs can be formed as early as in the crystal pulling process during the cooling of the single crystal. If the nuclei exceed a temperature-dependent critical size, they are able to grow during a thermal treatment. These BMD nuclei capable of growth are referred to as stable nuclei.
The density of the BMD nuclei cannot be determined directly on account of their small size. In order to measure the density of the stable BMD nuclei, the finished silicon wafer which, however, has not yet been structured in the context of a component process, is usually subjected to a BMD test. This test may consist, for example, in holding the silicon wafer at a temperature of 780° C. for three hours and subsequently at a temperature of 1000° C. for 16 hours. During this thermal treatment, stable BMD nuclei are stabilized further in the first step in order that they can grow to form large detectable BMDs in the second step at 1000° C. within 16 h. Detection takes place after the thermal treatment by means of Secco etch at a fracture edge of a broken silicon wafer in the case of an etching removal of 2.5 μm. This is a customary test for examining the oxygen precipitation behavior of silicon wafers. In another BMD test which is often used, and which leads to similar results, the silicon wafer is held at a temperature of 800° C. for four hours and subsequently at a temperature of 1000° C. for 16 hours.
The stable BMD nuclei that grow to form large BMDs as a result of thermal treatment can impair the functions of the electronic circuits and components by e.g. producing short circuits or reducing the lifetime or the number of the electrical charge carriers within the silicon wafers.
This problem has generally been solved hitherto by means of a thermal treatment that leads to a denuded zone at the surface of the silicon wafer. US2008/0292523A1 describes several methods for producing such a denuded zone. In one case, the silicon wafer is heated to a temperature of above 1000° C. in a short time (a maximum of 100 ms) by means of halogen lamps, xenon flashlamps or a laser and is then rapidly cooled again. The BMD nuclei are thereby eliminated in a thin layer below the surface. Stable BMD nuclei still exist, by contrast, at a depth of greater than 10 μm. After a flashlamp heat treatment for a duration of 1 ms at a maximum temperature of 1250° C., the BMD density within the silicon wafers is 3.8·106 cm−2 (corresponding to approximately 1.9·1010 cm−3), and the thickness of the denuded layer is 0.6 μm. At a maximum temperature of 1300° C., the result is a denuded layer having a thickness of 0.8 μm, and a BMD density of 5.2·106 cm−2 (corresponding to approximately 2.6·101° cm−3) in the rest of the silicon wafer. The BMD density was measured after a thermal treatment at 800° C. for four hours and subsequently at 1000° C. for 16 hours.
For components for which a short lifetime of the charge carriers within the silicon wafer such as is caused by the BMDs is harmful, silicon wafers having a high BMD density internally and only a thin denuded zone at the surface are unsuitable, however.
Therefore, methods have also been developed which make it possible to free the entire volume of the silicon wafer of BMD nuclei. U.S. Pat. No. 6,336,968B1 describes a method wherein a silicon wafer is heated rapidly to a temperature of at least 1150° C. and remains at this temperature for a number of seconds (at least 1 s) in order to dissolve the pre-existing BMD nuclei. Afterward, the silicon wafer is cooled at a cooling rate of at most 20 K/s to a temperature of a maximum of 950° C. At the maintenance temperature of at least 1150° C., a very high concentration of crystal lattice vacancies arises, these vacancies normally becoming supersaturated during cooling and greatly promoting the origination of new BMD nuclei. By means of the slow cooling, they are intended to be outdiffused beforehand. The same effect can be achieved by keeping the wafer at a constant temperature in the range of 1150 to 950° C. for longer (e.g. >2 s at 1150° C. or >2 min at 950° C.). The reduction of the vacancy supersaturation can be supported by an oxygen-containing atmosphere because the oxidation of the surface generates silicon interstitials (interstitial silicon atoms) which recombine with the vacancies and thus reduce their density further. The problem of this method is that the vacancies bind to oxygen at temperatures below 1150° C. and their outdiffusion is thus made significantly more difficult because the back reactions which liberate the vacancies again require a certain time. The method according to U.S. Pat. No. 6,336,968B1 therefore requires a comparatively long time for processing.